Microbial Growth Curve Framework Provides Insights for Controlling Opportunistic Pathogens in Building Plumbing

Area of Science:

• Environmental microbiology
• Building water systems
• Public health

Background:

• Building water systems can harbor microbial growth, including pathogens like Legionella pneumophila.
• Current practices often involve costly flushing interventions based on the assumption that high water retention time (WRT) increases risk.
• A scientific framework for guiding flushing interventions is lacking.

Purpose of the Study:

• To evaluate the effect of WRT on microbial growth in building plumbing systems.
• To develop a framework for understanding microbial growth dynamics in relation to WRT and control measures.
• To assess the efficacy of current flushing guidelines.

Main Methods:

• An at-scale building plumbing rig was used to simulate WRT from 1 to 21 days.
• Experiments were conducted with varying chloramine residuals (<0.2-2.5 mg/L) and water heater temperatures (40 °C and 60 °C).
• Total cell counts (TCC) and Legionella pneumophila levels were monitored over time.

Main Results:

• Microbial growth followed a classic curve (lag, exponential, stationary, decay) across tested conditions.
• Bulk water TCC and L. pneumophila peaked at approximately 7 days WRT before decaying significantly (up to 90%) at longer WRTs.
• Disinfectants and pasteurization temperatures acted to inhibit or reset microbial growth curves.

Conclusions:

• The classic microbial growth curve effectively explains microbial trends in building water systems.
• High WRT does not necessarily correlate with increased microbial risk; levels can decrease after a certain point.
• Weekly flushing recommendations may be counterproductive in some scenarios, and a WRT-based framework is needed.